EFFECT OF HYPOTHERMAL STRESS ON MILK-COMP OSITION AND HEALTH OF DAIRY-COWS

Thirty Holstein cows were matched by pairs by stage of lactation and m ilk yield. They were then randomly assigned to housing treatment group . The trial group (outside) was kept in an open free stall barn and th e control group (inside) in a warm enclosed tie stall barn. Milk yield was measured twice daily and milk composition twice monthly. Milk com position was determined in composite AM-PM samples. Fat, protein, lact ose and SNF were determined by Dairy Herd Improvement Laboratory using a Multispec M infrared analyser. Total mixing ration per cow and day consisted of 11.3 kg alfalfa haylage, 4.5 kg corn silage, 4.8 kg alfal fa hay, 16.7 kg concentrate mix and 0.9 kg molasses. An average of min imum temperatures in the open barn was lowest during the eighth (-19.4 degrees C) and during the seventh out of twelve weeks of the experime nt, the averages below -10 degrees C were recorded. Tab. I shows tempe rature regime on days of taking the milk samples. The minimum temperat ures in the open barn were lowest in sixth and eighth week (-19.1 degr ees C and -17.2 degrees C). A significant difference of 2.68 kg in mil k yield was observed throughout the whole experiment between the trial and control groups. In both groups, fat content had a similar trend. A significant improvement was recorded in the sixth week. This elevati on was statistically significant in the trial group in relation to the control group. It was that week when the lowest temperature was recor ded in both housing systems (Tab. II). The average fat content in dair y cows kept in the environment with lower temperatures was 0.117 kg hi gher than in the other type of housing. The protein content in the tri al group was significantly higher in the whole experiment. Contents of lactose and solids-non-fat in percentage had a similar trend with a r apid decrease in the sixth week. Concentrations of these parameters we re slightly lower in the trial group compared with control group (Tab. III). When transformed to kg, the results were opposite and an averag e content of the fat-free solids was shown as significantly higher in cows from the open barn (2.771 kg versus 2.567 kg). The average of the total solids content was also significantly higher in animals from th e open barn (4.132 kg versus 3.803 kg). Somatic cell count was also hi gher during the whole period of observation of the trial group (Tab. I II). The greatest differences were recorded on the sixth and eighth we eks. Closest relationships between the qualitative parameters of milk and feed intake were found in fat yield and total solids. Also in othe r important parameters, such as protein, lactose and solids-non-fat th ere were positive relationships except in the sixth week (Tab. IV). Co rrelation relationships between the stage of lactation and the yield o f milk, fat, protein, lactose, SNF and total solids were negative and above all significant in the control group (Tab. V). On the contrary, percentage yields of fat, protein and SNF were significantly positive with the exception of dairy cows from the open barn. Tab. VI shows eff ects of age at the first calving on the quality of milk. Yield of milk , protein, lactose, SNF and total solids is in a negative relationship with the age. This relationship was strengthened in the tenth and twe lfth week in both groups. Somatic cell count is in a positive relation ship with the age at the first calving. In the control group, signific ant relationships were recorded. We assessed more cases of mastitis an d diseases of legs in the trial group. On the contrary, more cases of reproductive diseases were in the control group.